Excitation of a surface wave on a thin plasma sheath surrounding a missile



ne i. 21, 1965 J. G. HOFFMAN 3,208,068

EXCITATION OF A SURFACE WAVE ON A THIN PLASMA SHEATH SURROUNDING A MISSILE Filed Sept. 24, 1962 1y II V JOHN G. HOFFMAN INVENTOR.

ATTORNEY United States Patent Of ice 3,258,068 Patented Sept. 21, 1965 EXCTTATION OF A SURFACE WAVE ON A THIN PLASMA SHEATH SURROUNDTNG A MISSILE John G. Hoffman, Riverside, Califi, assignor to the United States of America as represented by the Secretary of the Navy Filed Sept. '24, 1962, Ser. No. 225,941 7 Claims. ((11. 3437t 5) (Granted under Title 35, US. Code (1952), see. 266) The invention herein described may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

The present invention relates to waveguide apparatus and more particularly to means to couple a waveguide to a plasma sheath (i.e., ionized atmospheric layer) upon which is developed an electro-magnetic wave.

In any surface wave mode of propagation, the greatest problem is the exciting of this mode efiiciently or the coupling of a standard waveguide mode to a surface wave mode. Previous methods of excitation of a surface wave mode were by either from flared waveguide or probes onto the surface Wave structure. The inefficiency in couplings, where the efficiency in couplings are less than 60%, is a disadvantage of the old methods. The development of the present invention provides an eflicient method of exciting a microwave surface wave upon a plasma sheath or ionized atmospheric layer.

The important feature of the present invention is in the provision of a plurality of serially arranged Varactor diodes which span the height of a section of waveguide along the length thereof to adjust the propagation constant of the waveguide.

It is an object of the invention therefore to provide an efficient method of exciting a microwave surface wave upon a plasma sheath.

It is another object of the invention to provide means to couple a waveguide to a plasma sheath upon which is developed an electro-magnetic wave.

A further object of the invention is to provide efiiciency of coupling a waveguide to a plasma sheath with no independent radiation pattern that is characteristic of feed systems in surface wave structures.

Other objects and many of the attendant advantages of this invention will become readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein:

FIG. 1 is a perspective view of an embodiment of the invention showing a section of Varactor loaded waveguide with a coupling aperture to the plasma sheath.

FIG. 2 shows a cross-section of the waveguide of FIG. 1 taken along section line 22.

Referring to the drawings, like numerals refer to like parts in each of the figures.

A plasma slab or sheath is an ionized atmospheric layer in proximity of a hypersonic vehicle or antenna either man made or caused by a re-entry of a vehicle from space into the atmosphere at high speeds. It is this ionized layer that causes a period of communications blackout during re-entry. A more complete discussion of the plasma sheath is found in Proceedings of Symposium on the Plasma Sheath, vol. I, R. F. Turner- Organizer, December 7-9, 1959, AFCRCTR108(I), Air Force Cambridge Research Center, Bradford, Mass, also published by Pergamon Press.

Since the velocity of propagation of a surface wave in a plasma is much greater than in a standard waveguide mode and since the propagation constant in a surface wave mode is less than for a waveguide mode it is necessary, in order for any efliciency of coupling between the two mode structures that the propagation constants in the two types of structures should be approximately equal. Also, since the plasma electron density is constant and consequently the plasma sheath propagation constant, it is necessary to make modifications upon the waveguide structure to change its propagation constant.

The propagation constant of a waveguide structure 10 can be changed by the use of a plurality of Varactor diodes 12 placed parallel to the electric field in the waveguide. The waveguide structure illustrated is a section of standard rectangular waveguide 10, closed at one end 14, or other suitable waveguide configuration, and having a coupling slot aperture 16 along one broad surface thereof. The Varactor diodes 12 each have one terminal thereof electrically connected to the side of waveguide 10 having coupling aperture 16. Transverse slots, a series of coupling apertures or other suitable aperture configurations than shown may be used. Terminal leads 18 from the other side of Varactor diodes 12. each pass through a respective aperture 20 on a side of waveguide 10 opposite aperture 16. Varactor diodes 12 are connected in parallel and through leads 22 and 23 to a Varactor bias voltage source 24; the Varactor bias voltage being varied as a function of the plasma electron density.

The Varactor diodes 12 have the effect of varying the capacitance along the waveguide which in turn varies the propagation constant in the waveguide. Hence, efficient coupling from the standard waveguide mode to the plasma sheath surface wave mode adjacent the waveguide aperture is assured of coupling.

As shown in FIG. 1, a Varactor loaded waveguide has a coupling aperture 16 for coupling to a plasma sheath. By varying the Varactor bias voltage from source 24 the effective capacitance of diodes 12 change and consequently the propagation constant in waveguide 10 changes. If the Varactor bias voltage is a function of the electron plasma density, then efficient coupling to the plasma sheath is assured. The operation of the systern is as follows: Imagine a plasma sheath enclosing the side of waveguide 10 having the coupling aperture 16. The plasma sheath (i.e., ionized layer) is a conducting medium whose conductivity varies with its electron plasma density. The electron density of the plasma sheath (i.e., ionized layer) adjacent the slotted surface of antenna 10 can be determined by voltage probes 26 which extend into the conducting plasma since a bias voltage drop across the probes will result in a current flow between the probes when they are immersed in the conductive plasma, the amount of current flow being a function of the density of the plasma. The current flow which varies with the plasma density acts as a control for varying the Varactor bias voltage supply 24 which in turn adjusts the voltage across the Varactor diodes and thus the capacitance, to Where the propagation constant in the waveguide equals the propagation in the plasma sheath.

This present technique of excitation has the particular advantage of efliciency of coupling a waveguide to a plasma sheath and hence there is no independent radiation pattern that is characteristic of feed systems in surface wave structures. Other advantages are found in the controllable coupling between the exciting waveguide and the plasma sheath, and excitation of a plasma sheath as a surface wave structure.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than is specifically described.

What is claimed is:

1. A system for exciting a microwave surface wave upon a plasma sheath and for providing a controllable coupling between a waveguide and a plasma sheath, comprising:

(a) a section of waveguide,

(b) a coupling aperture along one side of said waveguide adjacent to a plasma sheath,

() a plurality of Varactor diodes positioned in said waveguide along the length thereof,

(d) means responsive to the electrical characteristics of the plasma sheath for applying an adjustable bias voltage across said Varactor diodes in a controlled amount as determined by the electron density in the plasma sheath to where the propagation constant in said waveguide equals the propagation in the plasma sheath.

2. A system for exciting a microwave surface wave upon a plasma sheath and for providing a controllable coupling between a waveguide and a plasma sheath, comprising:

(a) a section of waveguide having one end thereof closed,

(b) at least one coupling aperture along one side of 'said waveguide and adjacent a plasma sheath,

(c) a plurality of Varactor diodes positioned in said waveguide along the length thereof,

(d) means responsive to the electrical characteristics of the plasma sheath for applying an adjustable bias voltage across said Varactor diodes in a controlled amount as determined by the electron density in the plasma sheath to where the propagation constant in said waveguide equals the propagation in the plasma sheath.

3. A system for exciting a microwave surface wave upon a plasma sheath and for providing a controllable coupling between a waveguide and a plasma sheath, comprising:

(a) a section of waveguide,

(b) at least one coupling aperture along one side of said waveguide and adjacent a plasma sheath,

(0) a plurality of serially arranged Varactor diodes positioned parallel to the electric field in said waveguide and along the length thereof,

(d) means responsive to the electrical characteristics of the plasma sheath for applying an adjustable bias voltage across said Varactor diodes in a controlled amount as determined by the electron density in the plasma sheath to where the propagation constant in said waveguide equals the propagation in the plasma sheath.

4. A system for exciting a microwave surface wave upon a plasma sheath and for providing a controllable coupling between a waveguide and a plasma sheath, comprising:

(a) a section of waveguide,

(b) coupling aperture along one side of said waveguide adjacent to a plasma sheath,

(c) a plurality of Varactor diodes positioned in said waveguide along the length thereof,

(d) means for determining the electron density in the plasma sheath,

(e) means connected to said electron density determining means for applying an adjustable bias voltage across said Varactor diodes in a controlled amount as determined by the electron density in the plasma sheath to where the propagation constant in said waveguide equals the propagation in the plasma sheath.

5. A system for exciting a microwave surface wave upon a plasma sheath and for providing a controllable coupling between a waveguide and a plasma sheath, comprising:

(a) a section of waveguide,

(b) at least one coupling aperture along one side of said waveguide and adjacent a plasma sheath,

(c) a plurality of serially arranged Varactor diodes positioned parallel to the electric field in said waveguide and along the length thereof,

(d) means for determining the electron density in the plasma sheath,

(e) means connected to said electron density determining means for applying an adjustable bias voltage across said Varactor diodes in a controlled amount as determined by the electron density in the plasma sheath to where the propagation constant in said waveguide equals the propagation in the plasma sheath.

6. A system as in claim 5 wherein said means for determining the electron density of the plasma sheath is a pair of probes extending into the plasma sheath and connected to said means for applying a bias voltage to said diodes for controlling the amount of bias voltage applied.

7. A system as in claim 5 wherein one terminal of each diode is connected to the side of said waveguide having the aperture and the other terminal of each diode passes through a respective aperture in the opposite side of said waveguide and connected together in parallel and across said bias voltage means.

References Cited by the Examiner UNITED STATES PATENTS 3,076,149 1/63 Knechtli et al. 330-46 3,079,571 2/63 Elliott et al. 307885 3,094,664 6/63 Kibler 330-46 HERMAN KARL SAALBACH, Primary Examiner, 

1. A SYSTEM FOR EXCITING A MICROWAVE SURFACE WAVE UPON A PLASMA SHEATH AND FOR PROVIDING A CONTROLLABLE COUPLING BETWEEN A WAVEGUIDE AND A PLASMA SHEATH, COMPRISING: (A) A SECTION OF WAVEGUIDE, (B) A COUPLING APERTURE ALONG ONE SIDE OF SAID WAVEGUIDE ADJACENT TO A PLASMA SHEATH, (C) A PLURALITY OF VARACTOR DIODES POSITIONED IN SAID WAVEGUIDE ALONG THE LENGTH THEREOF, (D) MEANS RESPONSIVE TO THE ELECTRICAL CHARACTERISTICS OF 